The invention concerns the housing and recharging of batteries for battery powered devices such as portable power tools and certain kitchen and domestic appliances, and more particularly a system in which a contained group of power cells, commonly known as a battery pack, may be inserted into a device as a power source, and removed from the device and inserted into a charger assembly for recharging.
The several advantages of cordless power for tools and appliances have led to the development of a wide range of sizes of power- or battery-pack. Low powered units of two or three cells have long been known. Much larger sizes are becoming more common and there is already a potential demand for units up to say twenty cells. Preferably one charger should handle all sizes of battery pack.
It has been conventional to use chargers based on a step down transformer. The inherent isolation of charger output terminals and of batteries under charge from high charger input voltages is an advantage. But transformer chargers are essentially voltage sources and transformer size must be closely related to the number of cells to be charged. With some electronic circuitry the range of a given transformer charger can be extended somewhat, but this is not generally cost effective and a "universal" transformer charger, for covering a wide range of battery pack sizes, is not feasible.
The capacitive charger, essentially a current source, is not "cell dependent" in the way that a transformer charger is. Hence it has been a potential alternative or replacement for the transformer charger, but high capacitor cost has hindered its introduction. However, recent developments in capacitor design, notably the introduction of the metalized film capacitor, makes its use in charger design cost effective. But to realize the potential contribution of the capacitor to the design of a more truly universal charger, charger receptacle and terminal assembly design must preferably be made compatible with battery packs not only varying in power size but also in physical size and configuration. At the same time it must be recognized that the capacitor charger is inherently non-isolated so that the charger and battery pack terminals, and the cell terminals when under charge, are at high voltage.
Hence, it has continued to be common practice to provide a specific charger for each tool resulting in wasteful duplication. In U.S. Pat. No. 4,173,733, Sugalski suggests a combination in which one charger design accepts at least two configurations of holder, holding cells for charging. But Sugalski does not deal with battery packs per se and in his design, integrity or stability of the connection between the cell holder and charger depend entirely on the button and socket-type terminals connecting them, and at least before connection, the terminals on both cell holder and charger are exposed.
To adapt his charger to different battery packs, Busch, in German Patent DE 2702129, accepts the inconvenience of additional parts, providing a specific adapter for fitting each of two styles of battery pack to the charger receptacle. The charger terminals remain exposed at the bottom of the charger receptacle.
Important elements in the control of a charging process are, typically a device for sensing battery temperature (often a thermistor is used), and an associated charging indicator light. Normally the temperature sensor is closely associated with one or more of the cells of the battery pack and the entire charger/tool system is burdened with the cost of providing a temperature sensor and possibility indicator light for each battery pack, as in Sugalski. In another example, Hashimoto, in German Patent DE 3144858 (FIG. 2), discloses a thermistor within the battery pack. His configuration reguires an additional terminal on the battery pack and on the charger to electrically connect the thermistor with the charging circuit. Hernandez in U.S. Pat. No. 4,616,171 discloses a thermistor (FIG. 4a) projecting through an opening 120 in charger casing 121. Thermal contact with batteries 206 of the battery pack 200 is made through opening 212 (FIG. 6). This arrangement avoids the duplication of control elements in the battery packs but has the disadvantage of requiring potentially troublesome openings in the charger and battery pack housings.
It is well known to provide the openings of electrical power outlets with shutters or shields which impede accidental contact with their live terminals. See for example U.S. Pat. Nos. 2,579,538 Bierce and 4,493,517 Hillary, both of which disclose safety shutter arrangements in an outlet, normally responsive only to the insertion of a conventional mating plug. But similar provisions in charger/battery pack combinations are not known.